1. Field of the Invention
The present invention relates to a method and apparatus for treating material under pressure, and more particularly to a method and apparatus for cooling and freezing foods.
2. Description of the Prior Art
Cooling of foods, especially freezing of foods, is usually accomplished under atmospheric pressure using a freezing apparatus. The rate of cooling is determined by the thermal conductivity of food. This results in slow cooling or freezing, which in turn forms large ice crystals in food to impair its quality. For this reason, it has been difficult to freeze soybean curd, agar-agar, "konjak" (a hard jelly made from the starch of devil's tongue), and the like.
In order to overcome this difficulty, there has been proposed a method of freezing food by means of cooling and pressurizing in combination. (See "Shokuhin to Kaihatsu" (Foods and Development), vol. 26, No. 12, 1992.) This method is called a pressurizing-depressurizing freezing method. This method utilizes the unique property of water that water in food does not freeze under pressure. According to the above-cited reference, this method consists of placing a food (soybean curd) with a pressure medium in a pressure vessel provided with a jacket in which a cooling medium is circulated, pressurizing the contents of the vessel such that the food is isostatically compressed, cooling the vessel so that the temperature of the food reaches a temperature below 0.degree. C. but still above the freezing temperature under pressure, and rapidly depressurizing the contents of the vessel. In the pressurizing and cooling process, the pressure in the vessel increases to 1500 kgf/cm.sup.2 and the temperature in the vessel decreases to -13.degree. C. along the path (a) shown in FIG. 6. In the depressurizing process, the pressure in the vessel decreases along the path (b) shown in FIG. 6. The rapid pressure decrease in the vessel causes water in the food to shift from liquid phase to solid phase, forming fine ice crystals instantaneously. The resulting frozen food restores its original quality upon thawing, unlike the conventional frozen food produced by slow freezing.
The pressurizing-depressurizing freezing method is also disclosed in Japanese Patent Laid-open Nos. 216430/1985, 94664/1991, 148667/1992, 158772/1992, 271772/1992, and 49461/1993, in addition to the above-cited reference.
The pressurizing-depressurizing freezing method (conventional method) mentioned above has the disadvantage of requiring an extremely long time for cooling because the cooling along the path (a) is achieved by the circulation of a cooling medium through the jacket attached to the outside of the pressure vessel and heat transmission through the thick wall of the vessel is very poor. It is of no practical use although it is theoretically possible.